A typical structure of a conventional carbon arc lamp is illustrated in FIG. 1. As shown, two carbon electrodes are secured to a lower electrode holder, and one carbon electrode is secured to an upper electrode holder movable in a vertical direction inside the air-tight lamp housing consisting of a light-transmitting glass glove 1 and a substrate board 2. The upper electrode can be ignited and controlled by an iron core inside an electromagnetic coil 3 and a carbon suspension member 4 associated with said iron core.
Generally, the upper carbon electrode has a length of about 305mm and a diameter of about 13mm, and is either a core type or a coreless type. To make the lower carbon electrodes, a 305mm long carbon electrode, such as is used for the upper carbon electrode, has two lengths of 100mm cut therefrom, and the two thus formed electrodes are used as the lower carbon electrodes. The two electrodes are discharged alternately for a continuous combustion period of from about 20 to 22 hours.
As shown in the sectional views of FIGS. 2 and 3, the conventional carbon electrode has been either a coreless type (FIG. 2) or a core type (FIG. 3). The coreless type electrode is made predominantly of an amorphous carbonaceous material, such as carbon black or the like, whereas the core type electrode uses the abovementioned amorphous carbonaceous material for the exterior cylindrical section and a mixture of a stabilizing agent, such as potassium phosphate and the amorphous carbonaceous material for the interior core section.
A piece of material to be tested is mounted on a frame which rotates slowly around the arc lamp, and the piece of material is continuously irradiated by ultraviolet rays from the lamp over a period of several hundred hours to determine the light-fastness thereof.
Since the conventional carbon electrode has a life of only about 20-22 hours for continuous lighting, it would be desirable to extend this life to at least twice this time.
It is evident that if the length or the diameter of the carbon electrodes was increased, the combustion time could naturally be extended. An increase in the length of the carbon electrodes, however, is not economical because it also increases the overall dimension of the lamp apparatus. Although an increase in the diameter of the carbon electrodes does not cause any substantial change in the overall dimensions of the apparatus, stabilized light cannot be obtained if the diameter of the conventional carbon electrodes is increased unless the material is also changed. As shown in FIG. 4, for example, carbon electrodes having such an enlarged diameter do not always discharge from the tip thereof, but instead may discharge from the peripheral portion spaced from the tip. Likewise, the wearing of the carbon electrodes is not always uniform.
In the light of the teachings of my copending U.S. patent application Ser. No. 598,076 filed July 22, 1975, one of ordinary skill in the art would perhaps attempt to replace the carbon electrodes of the conventional lamp with upper and lower carbon electrodes which incorporate a mixture of a carbonaceous material and an illuminating agent as the core. When such carbon electrodes are used in practice in an ultraviolet arc lamp, however, discharge is effected from points spaced from the tip of the upper electrode as shown in FIG. 5. Thus, the discharge of this type of arc lamp has been found to be very unstable in practice.
Presumably this unstability results from the fact that because the carbon in said electrodes in said copending application is for use in an arc lamp for producing light similar to sunshine, and the carbon is burned in an atmosphere wherein oxygen is present, the carbon in the electrodes of the present invention is for use in an arc lamp for producing ultraviolet light, and hence, the combustion takes place in the absence or substantially complete absence of oxygen.